As form factor of many modern telecommunications devices shrinks, the design constraints for size of antennas increases. Mobile battery-powered devices in particular require both small size and energy efficiency. Antennas affect both the size and efficiency of these devices. In addition to size and power or radiation efficiency, other design goals for communication antennas may include directionality, higher bandwidth (lower Q), and manufacturing cost.
Two-dimensional microstrip antennas are attractive for modern devices for both their small size and low cost for manufacturing. Dimensions of two-dimensional antennas are often close to a quarter wavelength, and hence small, and they may consist simply of printed stripes of metal on an ordinary circuit board, though other materials and manufacturing methods are possible such as Teflon or alumina substrate.
A more efficient transmitting antenna will convert a larger portion of the energy fed to it into electromagnetic radiation, while a more efficient receiving antenna will convert a larger portion of received electromagnetic radiation into an electrical signal for processing by receiving electronics.
Simple loop antennas are typically current fed devices, which produce primarily a magnetic (H) field. As such, they are not typically suitable as transmitters. This is especially true of small loop antennas with an electrical length of less than one wavelength at the target frequency of usage. In contrast, voltage fed antennas, such as dipoles, produce both electric (E) fields and H-fields and can be used in both transmit and receive modes.
The amount of energy received by, or transmitted from, a loop antenna is, in part, determined by its area. Typically, each time the area of the loop is halved, the amount of energy which may be received/transmitted is reduced by approximately 3 dB depending on application parameters, such as initial size, frequency, etc. This physical constraint tends to mean that very small loop antennas cannot be used in practice.
Electrically short (ELS) antennas, as defined by H. A. Wheeler, are antennas with dimension very small as compared to the wavelength radiated from or received by them. The size of ELS antennas are attractive for small form-factor devices. However, ELS antennas suffer from large radiation quality factors, Q, in that they store, on average, more energy than they radiate. Such high Q results in a small resistive loss in an antenna or matching network and leads to very low radiation efficiencies, typically 1-50%, and narrow bandwidths.
Compound field antennas are those in which both the transverse magnetic (TM) and transverse electric (TE) modes are excited. In contrast to both simple loop antennas and ELS antennas, compound field antennas can achieve higher performance benefits such as higher bandwidth (lower Q), greater radiation intensity/power/gain, and greater efficiency. Designing a compound field antenna has often proven difficult due to the unwanted effects of element coupling and the related difficulty in designing a low loss passive network to combine the electric and magnetic radiators.
The basis for the increased performance of compound field antennas, in terms of bandwidth, efficiency, gain, and radiation intensity, derives from the effects of energy stored in the near field of an antenna. In RF antenna design, it is desirable to transfer as much of the energy presented to the antenna into radiated power as possible. The energy stored in the antenna's near field has historically been referred to as reactive power and serves to limit the amount of power that can be radiated. Complex power refers to separate real and imaginary components of power, where the imaginary component is often referred to as the “reactive” portion. Real power leaves the source and never returns, whereas the imaginary or reactive power tends to oscillate about a fixed position (within a half wavelength) of the source and interacts with the source, thereby affecting the antenna's operation. The presence of real power from multiple sources is directly additive, whereas multiple sources of imaginary power can be additive or subtractive (canceling). The benefit of a compound antenna is that it is driven by both TM (electric dipole) and TE (magnetic dipole) sources at the same frequency which allow engineers to create designs utilizing reactive power cancellation that was previously not available in simple field antennas, thereby improving the real power transmission properties of the antenna.
In order to cancel reactive power in a compound antenna, it is necessary for the electric far field zone and the magnetic far field zone to operate orthogonal to each other. While numerous arrangements of the electric field radiator(s), necessary for emitting the electric field, and the magnetic loop, necessary for generating the magnetic field, have been proposed, all such designs have invariably settled upon a three-dimensional antenna until U.S. Pat. No. 8,149,173 introduced a compound loop (CPL) antenna in planar configurations, that operated with compound antenna efficiency provided the electric filed radiator was connected to the magnetic loop at a 90 or 270 degree phase difference location on the magnetic loop.
While the concept of image theory makes it possible to reduce the size of the artwork for an antenna by half, if the antenna is completely symmetrical, by replacing half of the antenna with a ground plane, it has not been possible to implement image theory with a CPL antenna because the 90 or 270 degree location requirement resulted in electric filed radiator being placed in a position where a symmetrical design was not possible. And, while certain antennas may look the same as a symmetrical CPL antenna, such as an antenna illustrated and described in “Dual-Band Loop-Dipole Composite Unidirectional Antenna for Broadband Wireless Communications,” Wen-Jun Lu, et al, in IEEE Transactions on Antennas and Propagation, vol. 62, no. 5, pp. 2860-2866, May 2014, the dipole located inside the loop of the antenna operates at a different frequency than the magnetic loop and therefore cannot be a CPL antenna.